1. Field of the Invention
The present invention generally relates to a method and apparatus for detecting the gear position of a transmission for a vehicle having an engine, and more particularly to a microcomputer based method and apparatus for detecting the gear position of a transmission in response to sensed engine speed and vehicle speed.
2. Description of the Prior Art
The gear position of a vehicular transmission system is detected or ascertained and the detected gear position is used as an operating parameter in various kinds of control systems, such as a fuel supply control to an internal combustion engine.
Fuel cut off control systems are currently in use as a part of a fuel supply control for internal combustion engines, wherein the fuel supply is cut off, usually in the top gear position only. The reason for this is that if the fuel supply is interrupted during operation in gear positions other than the top gear, the shock inherent upon entering into the fuel cut off mode is considerable and causes discomfort to the vehicle's occupants.
Therefore, whether the transmission is in fact in top gear should be determined positively so as to eliminate the above-mentioned problem.
In a previously proposed gear position detecting system a plurality of mechanical switches, such as microswitches, are attached to suitable portions of the transmission which are actuated in order to achieve a given gear position, and arranged to produce an electric signal indicative of the selected gear position.
However, this detecting system suffered from the disadvantage that the step of attaching the microswitches to the appropriate portions of the transmission undesirably adds to the number of steps during assembly of the transmission. Furthermore, after long use, the microswitches are apt to malfunction.
In order to eliminate the above mentioned disadvantages, there has been proposed a microcomputer based system for detecting the gear position of a transmission.
The system involves a mathematical operation or process based on sensed engine speed and sensed vehicle speed (which can be represented by the rotational speed of a transmission output shaft).
The operation of the system is set forth hereinafter.
When the vehicle is running, the following relationship pertains: ##EQU1## where: V(km/h) represents the vehicle speed, N (rpm) represents the engine speed, G.sub.t represents the transmission gear ratio, G.sub.0 represents the final gear ratio, and r (m) represents the effective radius of a tire.
From equation (1), the ratio N/V can be expressed as follows: ##EQU2##
Assuming that the gear ratio G.sub.t of the top gear is 1.0 and the gear ratio of the third gear is 2.0, equation (2) can be expressed as follows:
When the top gear is selected; ##EQU3## and when the third gear is selected; ##EQU4##
In order to mathematically detect the gear position, the engine speed N, for example, is compared with a reference level K obtained by multiplying the vehicle speed V by the mean value k of the above described N/V values associated with each gear position.
In this case, the reference level K is determined to be ##EQU5##
If the engine speed N is lower than the reference value K (N&lt;K), the system determines that the top gear has been selected.
Conversely, if the engine speed is higher than the reference level K (N&gt;K), the system determines that third gear has been selected.
Needless to say, the same result can be obtained by comparing the ratio N/V with a reference level k=75 G.sub.0 /6.pi.r which corresponds to the mean value of the N/V values as determined by equations (2) and (3) in place of comparing the vehicle speed V with the reference level K.
In the case of a centralized control system using a single microcomputer, however, the system is preferably arranged to operate on a time sharing basis. Therefore, the input data such as the engine speed data and the vehicle speed data are intermittently inputted to the microcomputer.
However, in view of the requirements of the computation of the other control modes such fuel supply control, vehicle speed data is supplied to the microcomputer at longer intervals than the engine speed data.
Therefore, when a rapid change occurs in the vehicle speed, such as the case of an abrupt deceleration, the vehicle speed data read in by the microcomputer may not accurately follow with the actual vehicle speed, and consequently, it is likely that the system make an erroneous determination of the gear position.